Electrostatic Discharge (ESD) is a known problem in the manufacturing and the using of integrated circuits. Typically, transistors have thin oxides and insulating layers that can be damaged by ESD events, and special care is required to protect the integrated circuits from the damage caused by the ESD events.
In integrated circuits, there are various types of devices, including logic devices, Input/output (IO) devices, high-voltage devices, and the like. Different types of devices may require different power supply voltages. In an application that includes different types of devices, accordingly, a plurality of power rails, with each carrying one of the power supply voltages, is needed. The power rails need to be protected by ESD clamps, which are used to discharge the ESD currents in response to the ESD transients.
Due to the use of multiple power supply voltages, the gate dielectrics of the MOS devices in the ESD clamps need to be able to endure the highest voltages that may be applied thereon without being broken down. Accordingly, the ESD clamps need to use thick gate oxides. The use of the thick oxides, however, results in the increase in the manufacturing cost sine there are other devices that need to use thin oxides. Although capacitors and diodes may also be used in the ESD clamps to avoid the use of dual gate oxides, the capacitors and the diodes, however, either require large chip areas or result in the degradation in the protection ability of the ESD clamps.